High quality semiconductors can be manufactured by molecular beam epitaxy (MBE) wherein atomic or molecular beams of the semiconductor constituents are directed at an epitaxial growth surface. Elements of importance in semiconductor devices include the members of Groups II, III, IV, V and VI of the periodic table. The most common sources of molecular beams for use in MBE are effusion ovens for sublimating solid elements. Most of the elements sublime as atoms but some, most notably elements of Groups V and VI, sublime as polymers, such as dimers, tetramers, or mixtures thereof. It is to the elements which sublime as molecules, and most particularly to arsenic, that the present invention relates.
For elements which sublime as molecules, the sublimator can be followed by a cracker, a second heated stage, which breaks the molecules into smaller molecules. The Group V elements, P, As and Sb, sublime as tetramers and are cracked into dimers. The Group VI elements S and Se sublime as polymers with n=2-8 atoms and are cracked into dimers. The Group V element Bi and the Group VI element Te sublime as dimers. In a typical two-stage cracking furnace for arsenic (see, for example, Lee et al., J. Vac. Sci. Technol. (1986) B4:568; Garcia et al., Appl. Phys. Lett. (1987) 51:593) the first stage is a metallic arsenic sublimator heated to about 600K, which produces a molecular beam consisting mainly of arsenic tetramers. The tetramer beam then enters the second stage, the cracker, which is at a temperature of about 800-1300K and a pressure of about 10 mtorr. The cracker walls are made of materials such as graphite, tantalum, boron nitride, molybdenum or rhenium. In the second stage the arsenic tetramers are cracked into dimers. The dimer yield increases with cracker temperature and depends on the cracker material.
A second type of molecular beam source starts with gaseous hydrides, as opposed to solid elements, and cracks the hydrides in a heated furnace. Arsenic molecular beam sources (see, for example, Calawa, Appl. Phys. Lett. (1981) 38:701 or Kapitan et al., J. Vac. Sci. Technol. (1984) B2:280) utilize AsH.sub.3 and a cracking furnace heated to about 800-1300K. The furnace is typically quartz and can also comprise heated tantalum. The resulting molecular beam contains H.sub.2, As.sub.4, As.sub.2, As and AsH and AsH.sub.3. In some cases the primary constituents are H.sub.2 and As. The purity of material grown in these cases suggests that atomic As may be the preferred specie for the growth of GaAs. A complication with methods using AsH.sub.3 as starting material is that AsH.sub.3 is a highly toxic gas, requiring special safety considerations. In addition, since these methods generate a high background of molecular hydrogen, high speed vacuum pumps are required.